Optical filter is generally used to select wavelength. The optical filter can be used to select lightwave with required wavelength from lightwave with a variety of wavelength. The optical filter is becoming more and more important in fiber optic communication systems.
Optical filters have many applications in wavelength division multiplexing (WDM) systems, including but not limited to optical spectrum analysis, laser testing, wavelength demultiplexing, and amplified spontaneous emission filtering. Traditionally, optical filters have been based on fibre Bragg gratings (FBGs), thin film dielectric interference filters, Fabry-Perot filters, phased array waveguides, and diffraction gratings. The ideal optical filter is usually associated with flat channel passbands, low insertion loss, high isolation, low cost, high reliability and field ruggedness, high thermal stability, and good filter roll-off characteristics. Furthermore, in many instances it is preferable that the filter is tunable.
Referring to FIG. 3, a tunable filter 10 used in optical signals having a wavelength of 100 GHZ ITU is provided. The tunable filter 10 includes an input optical fiber 113, an output optical fiber 111, a collimating/focusing element 13, a grating 15, and a reflector 17. Optical signal transmitted from the input optical fiber 113 passes through the focusing element 13 and is changed into parallel optical signal, which incidents on the grating 15 and be diffracted by the grating 15. Then the optical signal incidents on the reflector 17 and be reflected by the reflector 17 and returns in original path. Lastly, the optical signal is input into the output optical fiber 111. The optical signal is input via the input optical fiber 113 and lastly enters into the output optical fiber 111 by rotating the grating 15 or the reflector 17, The grating 15 is rotatable, so as to change the angle of incidence, and thus change the wavelengths that are filtered
However, a tunable filter used in optical signals having a wavelength of 50 GHZ ITU adopts the same optical path as that of a tunable filter 10 used in optical signals having a wavelength of 100 GHZ ITU, which results in using lens with larger diameter. In addition, the filter will have a larger size on the reflector, and therefore it is required to using reflector with larger diameter. Facular having a larger size will result in larger size grating. Therefore, the size of the filter became larger, which will decrease the general using of the filter. Moreover, the total cost of the filter will be increased.
Therefore, it is an object of the instant invention to provide an a tunable filter to overcome the above-described shortcoming.